Plastic films are characterized by such properties as good tensile strength, high ultimate elongation, good impact strength and puncture resistance. In addition to these characteristics the films need to exhibit esthetic qualities such as a smooth surface appearance. These properties together with toughness are relatively easily obtained when the polymer resin used to produced the films is polyethylene. However, as the molecular weight of the polyethylene increases, the processability of the polyethylene usually becomes extremely difficult, often requiring excessive energy inputs in a film-forming extruder. To improve processability, particularly extrudability in a film-forming extruder, a blend of polymers, such as a polyethylene and an alpha olefin, is often employed. One of the main reasons for blending is to obtain a resin having a set of desired properties such as strength in the fabricated article or film while allowing easy processability in the fabricating equipment. Blends of a low molecular weight polymer and a high molecular weight polymer, such as blend of a low molecular weight polyethene and a high molecular weight polyethene each having a narrow molecular weight distribution (i.e., a bimodal blend) are a particularly attractive route to achieving easy processability while increasing the strength of the film or fabricated article.
Three major strategies have been proposed for the production of polymer resins used to fabricate films. One is the direct production of a resin or a blend of resins having the above-mentioned characteristics of toughness and easy processability, via one or more catalysts in a single fluidized bed reactor. A second makes use of multi-stage reactors operating independently or in series, primarily to produce resin blends. A third strategy is a post-reactor or post-multiple reactor treatment, such as melt blending a resin or a blend of resins in an extruder.
In general, polymer resins manufactured employing the above-mentioned strategies require a certain degree of homogenization prior to being used to make a plastic film. This homogenization is typically carried out in a melt phase operation which involves mechanical mixing. The melt mixing is carried out in either batch mixers or continuous mixers such as single or twin screw extruders. The objective of the mixing step is to disperse and distribute one component of the blend into the other. Usually, the major component forms a matrix while the minor component forms a dispersed phase, although phase inversions are known to occur depending on the viscosity and elasticity ratios of the two components. The most useful commercial blends consist of a high molecular weight matrix with a low molecular weight component being in the dispersed phase. The high molecular weight matrix to a large degree determines the properties of the fabricated article in the solid state such as film strength, while the dispersed low molecular weight component aids in the processing of the blend.
In post treatments, such as in melt blending, the polymer resin is not readily mixed such that it becomes homogeneous. In plastic films, the inhomogeneity shows up as gels (also known as "gelling" or "gel-streaking"). Gel-streaking in films is an optical degradation phenomena evidenced by surface roughness in the form of the inclusion, either singularly or in rows, of v-shaped imprints (also referred to as "arrowheads" or "chevrons"), streaks of soft gels; or, in extreme cases, the film texture is completely permeated by soft gels. Such visual degradation phenomena not only detract from film optical and mechanical strength properties, but also can cause severe problems with respect to maintaining bubble geometry during a film forming process. Therefore, there is an on-going need for a compounding process to decrease the size and the number of gels in a polymer resin or resin blend used to fabricate film.
In commercial practice, post treatment blending processes usually involve the use of intensive mixing and prolonged mixing times. Additionally, to prevent or reduce gels, post treatment extrusion can require that a polymer be processed or passed through an extruder more than once. Both intensive mixing at prolonged times and multi-pass extrusion typically degrades the resin. Accordingly, there exists an on-going need for a post treatment extrusion process for polymer and polymer blends, especially polyethylene and polyethylene blends, which results in minimal or reduced gel formation in a finished plastic film.